The present invention generally relates to wireless communication systems, and particularly relates to interference suppression in wireless communication receivers.
Direct-sequence spread-spectrum modulation is commonly used in CDMA systems (such as W-CDMA, IS-95, and IS-2000). Data to be transmitted via a spread spectrum carrier is mapped into information symbols, and each information symbol is transmitted as a sequence of chips, which gives rise to bandwidth spreading. The sequence of chips used to spread the transmit symbols is referred to as the spreading code.
At the receiver, the received signal is despread using a despreading code, which typically is the conjugate of the spreading code. RAKE receivers represent a traditional approach to demodulating CDMA signals. RAKE receivers capitalize on the multipath propagation that typically exists between the transmitter and the receiver. Multipath propagation of the transmitted signal can lead to time dispersion, which causes multiple resolvable echoes (or rays) of the signal to be received at the receiver. A RAKE receiver aligns different ones of its “fingers” (correlators) with different ones of the signal echoes, and each finger outputs despread values at the symbol rate. These despread values are then weighted by the conjugate of the respective channel coefficient and then summed to produce a soft estimate of the transmitted symbol. This weighting and summing process is commonly referred to as RAKE combining.
Combining multipath echoes in the above manner yields an improved Signal-to-interference Ratio (SIR) when white noise is the dominant received signal impairment term at the receiver. However, RAKE combining is less than optimal when colored noise is the dominant impairment term. Colored noise arises from self-interference (Inter-Symbol Interference or ISI) and Multi-user Access Interference (MAI). As wireless networks crowd more and more users onto the same spectrum, and as the signal data rates increase, colored noise may become more problematic.
Thus, receiver structures capable of colored noise suppression represent an area of increasing interest. Unfortunately, the conventional approaches to interference suppression in colored noise environments entail potentially significant receiver complexity. For the typical portable communication device, such as a cellular radiotelephone, Portable Digital Assistant (PDA) or wireless pager, that complexity adds cost and undesirably affects cost, design time, and battery life.